Battery manufacturers are continuing to increase energy density, power characteristics, cycle life and safety characteristics, while decreasing the cost, of anode materials. Flaky natural graphite can be used as an anode material by transforming it into spherical particles through a process called spheroidization. Crystalline flake graphite (or flake graphite for short) occurs as isolated, flat, plate-like particles with hexagonal edges if unbroken and when broken the edges can be irregular or angular. Spheroidization of flake graphite can be accomplished by repeated pulverization of the flake graphite with a relatively small impact mill such as an air classifier mill (ACM) or pin mill. When pulverization is conducted, it is necessary to repeat the operation at least 10 times to produce a spherical shape with a high powder density. In order to obtain the targeted particle size (e.g., about 8-20 μm average), the average diameter of raw and flaky natural graphite should be about 1 μm to 50 μm. This finalized spheroidized powder can be used as an anode active material in a battery, for example, a lithium ion battery. Spheriodized graphite powder is typically a high surface area material that is useful to make anodes for high power, i.e., low impedance or low direct current resistance, battery applications. However, the efficiency of first charge and discharge of an anode made from this material is usually too low to achieve promising cell capacity. In addition, high temperature storage performance, i.e., good calendar life, is also poor for such anodes. The high surface area graphite powder does not perform well as at higher temperatures because of the loss of cyclable lithium generated from the solid electrolyte interphase (SEI) film reformation with electrolyte during cycling, i.e., during operation, and high temperature storage.
Instead, in order to increase the durability, i.e., cycle performance, of the natural graphite powders, the graphite is coated with pitch, which reduces the surface area of the graphite powder, but also increases the durability of the anode. Pitch is the name for any of a number of viscoelastic, solid polymers. Pitch can be made from petroleum products or plants. Pitch coated natural graphite is widely used for consumer battery applications such as laptops and cell phones. Pitch coated natural graphite is used because of its durability and low cost. Pitch coated graphite powder is good for cycling at low rate and calendar life (long term high temperature storage) because the pitch coating can minimize the side reactions with the electrolyte during cycling and high temperature storage. However, the pitch coating reduces the surface area of the active sites on the anode particles.
Because of the reduced surface area of the pitch coated graphite, cycle performance of the pitch coated anode materials is not well suited to a high power automotive application. With pitch coated anode materials, the interface between the pitch coated layer and the core graphite deteriorates during high power cycling operations. Accordingly, slightly higher surface area graphite powder has been used for high power applications.
The surface area of the anode powder is an important part of a lithium ion battery cell which affects cycling, storage, and safety performances of the battery. Appropriate surface area of anode powder will allow rapid reaction with lithium ions in the electrolyte. A high surface area anode, such as spheroidized graphite also can be good for regeneration power and high discharge cycling. A low surface area pitch coated graphite anode material performs well in terms on durability, but is not ideal for a high power application. Thus, an anode powder that creates a durable anode for high power applications is needed.